1. Field of the Invention
The present invention relates generally to industrial vehicles and more specifically to a hydraulic control system for a fork lift truck or similar industrial vehicle.
2. Description of the Prior Art
FIG. 1 of the drawings shows a previously proposed hydraulic control circuit 1 for controlling the lifting and tilting of cargo carried on the forks of a fork lift truck (not shown). In this arrangement an electric motor 2 is used to drive a hydraulic pump 3 which supplies hydraulic pressure to manually operated control valves 4 and 5. Located upstream of the control valves is a pressure relief valve 6 which discharges any excess pressure that prevails within the delivery circuit 7. The control valve 4 is adapted to control the pressurization of mast tilt cylinders 8 while the control valve 5 is adapted to selectively pressurize the cylinder 9 via which the forks are raised and lowered on the mast. To conserve the batteries 10, microswitches 12 and 13 are provided and incorporated into an electric circuit 11 which energizes the electric motor 2 only when either of the plungers or spools 14 and 15 of the control valves 4 and 5 are moved away from their illustrated home positions in a manner to pressurize a servo cylinder or cylinders.
In this arrangement, to eliminate the shock which is prone to occur when either of the valves is moved from or returned to its home position, each of the manual control valves is provided with a neutral port N which functions to by-pass the flow of pressurized hydraulic fluid from the pump into a drain circuit 16.
However, this arrangement has still suffered from the drawback that the speed at which the forks are elevated or the mast tilted can not be manually varied as after only a slight movement of either of the plungers from their home positions in a direction to pressurize a cylinder or cylinders, pump pressure is freely supplied to the cylinders. Thus, the cylinders tend to move the forks or tilt the mast with a start which of course is highly undesirable from the view point of safely handling cargo (such as hazardous liquids, fragile goods etc.).